Early morning transfer from downtown area to Beijing Capital Airport.

I have an international flight leaving from the PEK T3 in a few months that departs at 07:25 am in the morning. I haven't found a hotel to stay at yet but I'll likely stay somewhere downtown. What is the best way to get to the airport earlier than 06:00 am in the morning from somewhere downtown, either a bus/shuttle/subway? I'll likely try to find a hotel close to where your advise leads me.